Heretofore, various type fuel injectors and fuel injection systems have been known in the prior art which are applicable to internal combustion engines. Of the many types of fuel injection systems, the present invention is directed to unit fuel injectors, wherein a unit fuel injector is associated with each cylinder of an internal combustion engine and each unit injector includes its own drive train to inject fuel into each cylinder on a cyclic basis. Normally, the drive train of each unit injector is driven from a rotary mounted camshaft operatively driven from the engine crankshaft for synchronously controlling each unit injector independently and in accordance with the engine firing order.
Of the known unit injectors of such fuel injection systems, there are two basic types of unit injectors which are characterized according to how the fuel is metered and injected. A first type to which the present invention is oriented is known as an "open nozzle" fuel injector because fuel is metered to a metering chamber within the unit injector where the metering chamber is open to the engine cylinder by way of injection orifices during fuel metering.
In contrast to the open nozzle type fuel injector, there are also unit fuel injectors classified as "closed nozzle" fuel injectors, wherein fuel is metered to a metering chamber within the unit injector while the metering chamber is closed to the cylinder of an internal combustion engine by a valve mechanism that is opened only during injection by the increasing fuel pressure acting thereon. Typically, the valve mechanism is a needle type valve.
In either case, the unit injector typically includes a plunger element that strikes the metered quantity of fuel to increase the pressure of the metered fuel and force the metered fuel into the cylinder of the internal combustion engine. In the case of a closed nozzle injector, a tip valve mechanism is provided for closing the injection orifices during metering wherein the tip valve is biased toward its closed position to insure that injection will take place only after the fuel pressure is increased sufficiently to open the tip valve mechanism.
The present invention is directed to the open nozzle type fuel injector, and more specifically to a unit injector fuel injection system that relies on pressure and time principles for determining the quantity of fuel metered for each subsequent injection of each injector cycle. Moreover, the pressure time principles allow the metered quantity to be varied for each cyclic operation of the injector as determined by the pressure of the fuel supplied to the metering chamber and the time duration that such metering takes place.
Examples of unit injectors of the open nozzle type are described in detail in U.S. Pat. Nos. 4,280,659 and 4,601,086 to Gaal et al. and Gerlach, respectively, both of which are owned by the assignee of the present invention. The injectors of Gaal et al. and Gerlach include a plunger assembly with a lower portion having a major diameter section that is slidable within an axial bore of the injector body and a smaller minor diameter section that extends within a cup of the injector body. The cup provides an extension to the axial bore which is smaller in diameter than the diameter of the axial bore that passes through the remainder of the injector body. During the metering stage of the Gaal et al. and Gerlach injectors, fuel is metered through a supply port into the axial bore at a point above the cup, and the fuel flows around the minor diameter section of the plunger assembly at the tip thereof thus metering a specified quantity of fuel into the metering chamber of the cup. A radial gap is provided between the minor diameter section of the plunger assembly and the inner wall of the bore within the cup. This gap facilitates the flow of fuel to the injector tip to be injected. Once the metering stage is completed, the plunger travels inwardly (defined as toward the engine cylinder of an internal combustion engine) so as to cause injection of the fuel from the metering chamber through the injection orifices.
The stage just after the fuel injection has been completed is known as the crush stage, wherein the plunger tip is held tightly against a seat of the cup by the associated drive train for the unit fuel injector. During this crush stage, fuel is trapped within the radial gap between the minor diameter section of the plunger and the inner wall of the bore within the cup. This quantity of fuel is known as the trapped volume.
It has been found by the inventors of the present invention that this trapped volume results in the presence of higher levels of unwanted emissions, particularly unburned hydrocarbons. Moreover, the undesirable hydrocarbon emissions associated with open nozzle injectors have been found to be a function of the trapped volume within the nozzle, wherein excess volume increases the level of the unburned hydrocarbons. The increase in unburned hydrocarbons found in the emissions is due to the tendency of the fuel within the trapped volume to migrate into the engine cylinder after the combustion in the cylinder to be exhausted therefrom. Furthermore, the major component of the trapped volume results from the gap between the minor diameter section of the plunger and the inner wall of the cup. The area of this gap is commonly referred to as the labyrinth seal clearance region of the fuel injector.
As can be understood from the above, such a problem is unique to open nozzle type fuel injectors because closed nozzle fuel injectors rely on a valve mechanism to seal the fuel from the engine cylinder at all times except during injection. Moreover, open nozzle injectors must allow the metering of fuel within the nozzle tip with injection orifices that are open to the engine cylinder.
Thus, in order to reduce the trapped volume surrounding the minor diameter section of the plunger within the cup after injection, the only solution suggested by the prior art technology is to simply reduce the radial gap between the minor diameter section of the plunger and the cup to thus reduce the trapped volume after injection is completed. However, such a modification becomes unacceptable and results in the problem that there is no longer a sufficient gap for the fuel to be metered into the nozzle area of the cup since the fuel flow around the minor diameter section of the plunger becomes significantly reduced as the gap is reduced. Specifically, it has been found that the quantity of metered fuel to be injected is reduced to a degree that insufficient fuel is injected. Therefore, such a solution is impractical and unacceptable.
To make the situation worse, the components of the injector, specifically the plunger minor diameter section and the inner surface of the bore within the cup, become carboned during the usage of the unit fuel injector in an internal combustion engine from hot gases within the engine cylinder that are forced back into the injector. Furthermore, as carbon builds up on the minor diameter section of the plunger and the inner wall of the cup, the gap between the minor diameter section and the cup inner wall is effectively reduced during use. Thus, the effect of carboning on the injector elements tends to urge a designer to make the injector with a greater gap between the minor diameter section of the plunger and the inner wall of the cup so that even after carboning, sufficient flow can be provided through the gap for adequate fuel metering.
It is clear from the above that the known teachings to reduce trapped volume and to permit fuel metering without effect from injector carboning are in direct conflict with each other. In other words, reducing the trapped volume teaches decreasing the gap between the minor diameter of the plunger and the cup inner wall, while reducing the sensitivity to fuel metering after carboning requires the increase in gap size. The end result of the known open nozzle type unit fuel injector technology is that the above noted goals must be balanced with one another to provide a compromised open nozzle type unit fuel injector that has a gap that partially achieves both goals. Thus, it can be seen that such open nozzle fuel injectors are absolutely limited in their ability to reduce engine emissions while permitting adequate and effective fuel metering.
Another serious problem that is unique to open nozzle-type unit fuel injectors is the sensitivity of fuel metering to carboning of the unit fuel injector. Injector carboning occurs on all of the surfaces of the minor diameter section of the plunger and the inner surface of the cup. As best understood, the carbon forms as a result of essentially oil, fuel, and the temperature in the unit injector metering chamber. Moreover, carboning occurs during certain engine operating conditions wherein little or no fuel is present in the metering chamber. Such conditions include a motoring condition where the engine is being driven from the vehicle drive train or a part-load condition. The lack of fuel in the metering chamber during a condition such as motoring allows the gas temperatures inside the metering chamber to become very high. This happens because when the plunger tip is retracted to unseat from the cup during motoring or part-load, gas and airborne carbon enter the metering chamber from the engine combustion chamber as forced through the injector spray holes. The gas is forced within the metering chamber due to the increase in cylinder pressure during the compression stroke under such conditions. Then, as the gas is compressed by the advance of the plunger as operated for injection, temperatures are created sufficient to form carbon on the surfaces from the residual fuel in the injector. A study of the carbon deposits on the plunger and cup has shown that, in :ross section, a first layer of deposits on the surfaces is related to fuel and acts as a kind of adhesive. The outer layer consists of hard black carbon deposits which result mostly from oil. This accumulation of deposits is responsible for creating another major problem of open nozzle-type unit injectors in that the deposits create injector flow loss which inhibits the flow of fuel into the metering chamber during metering.
During metering, fuel must pass between the minor diameter section of the plunger and the inner wall of the cup to flow to the metering chamber at the cup tip. As the carbon deposits increase in thickness, the flow loss also increases. At some point it becomes impossible to obtain a sufficient fuel flow between the plunger minor diameter section and the cup inner wall such that a sufficient volume of metered fuel is created for injection. At this point, the unit injector cannot function properly.
Thus, in order to deal with the carboning situation, it has become necessary to replace, or at least service, such open nozzle unit fuel injectors after a period of running time, depending on operating conditions. As an alternative, efforts have been concentrated on reducing the formation of carboning as a means of lessening the effect of carboning on injector flow metering. However, once carboning eventually builds up, the injector will inevitably experience some injector flow loss.
For the above reasons, the popularity of closed nozzle fuel injectors has increased; however, the immediate disadvantage associated with closed nozzle fuel injectors is the extra costs that are associated with the production of such substantially more complex unit fuel injectors. Apart from the fact that a closed nozzle unit fuel injector functions on different operational principles than an open nozzle injector, as amplified above, closed nozzle injectors do not experience the same problems of open nozzle injectors enumerated above. Specifically, the valve of the closed nozzle injector does not have to be designed to accommodate precise metering at the nozzle while attempting to reduce trapped volumes. The only trapped volume that results within a closed nozzle type injector lies underneath a tip of a spring loaded nozzle valve just adjacent its injection orifices. Furthermore, injector carboning is not as prevalent in closed nozzle unit fuel injectors because the nozzle valve effectively closes the metering chamber to the engine combustion chamber during motoring or the like conditions.
An example of a closed nozzle fuel injector that specifically attempts to reduce the volume under the tip of the nozzle valve, noted as the SAC volume, is described in U.S. Pat. No. 4,106,702 to Gardner et al. In the Gardner device the tip of the nozzle valve is specifically tapered in a manner to reduce the SAC volume at the injection openings of the nozzle tip and to design the valve tip to seat against the interior conical surface of the nozzle. Although the Gardner et al. device is designed to reduce an SAC volume and reduce engine emissions related thereto, the closed nozzle type injector does not concern itself with reducing trapped volume in an environment that further must accommodate any metering of fuel for injection, since the nozzle valve simply reacts to the pressure of previously metered fuel and does not affect the metering of the injected fuel.
Other closed nozzle type fuel injectors including specifically designed nozzle valve tips can be found in U.S. Pat. Nos. 3,836,080 to Butterfield et al. 4,213,568 to Hoffman, and 4,523,719 also to Hoffman. Of these, the Butterfield et al. closed nozzle injector is further specifically designed to reduce the SAC volume under the nozzle valve tip. The design of Butterfield et al. is directed to solve the same problem of the Gardner et al. patent. Likewise, the problem attempted to be solved by Butterfield is not analogous to that within an open nozzle type injector wherein specific metering requirements must be met as well as reducing trapped volume and causing injection.
Thus, there is a need for an open nozzle unit fuel injector that can reduce trapped volume between the minor diameter of the plunger and the inner wall of the injector cup while still permitting sufficient fuel flow therebetween to accurately and effectively control the fuel quantity and reduce unburned hydrocarbons in the emissions. Moreover, there is a need to provide such an open nozzle unit fuel injector that will function accurately over the entire useful life of such an injector without adversely affecting fuel metering even after the plunger and cup surfaces become fully carboned.